1. Field of the Invention
The present invention relates to liquid crystal display (LCD) devices. More particularly, the present invention relates to a method for fabricating an In-Plane Switching (IPS) mode LCD device. More particularly still, the present invention relates to a method for fabricating an IPS mode LCD device wherein an alignment layer and an overcoat layer are formed substantially simultaneously.
2. Discussion of the Related Art
As information technology continues to evolve, the demand for, and development of, various types of flat panel display devices (e.g., liquid crystal display (LCD), plasma display panel (PDP), electroluminescent display (ELD), and vacuum fluorescent display (VFD)) increases. Among the various types of flat panel display devices, LCD devices are advantageously lightweight, dimensionally compact, consume relatively low amounts of power during operation, display images at high resolution and high luminance, and can display images on a large-sized screen. Accordingly, LCD devices are widely used, for example, as substitutes for Cathode Ray Tubes (CRTs) and find numerous applications in mobile devices as such notebook computers, portable telephones, and the like, as well as in other application such as televisions and computer monitors.
A typical LCD device includes an LCD panel for displaying images and a driver for supplying driving signals to the LCD panel. The LCD panel generally includes first and second substrates bonded to, but spaced apart from, each to form a cell gap therebetween. The first and second substrates are bonded together by a sealant material and a substantially uniform cell gap is maintained between the bonded substrates by the presence of spacers. A liquid crystal layer is formed within the cell gap between the first and second substrates by injecting liquid crystal material through an injection hole formed in the sealant material and into the cell gap.
Generally, the first substrate (i.e., the TFT array substrate) supports a plurality of gate lines spaced apart from each other at a fixed interval and extending along a first direction, a plurality of data lines spaced apart from each other at a fixed interval and extending along a second direction perpendicular to the first direction so as to cross the plurality of gate lines, a plurality of pixel regions arranged in a matrix pattern defined by crossings of the gate and data lines, a plurality of pixel electrodes arranged within the plurality of pixel regions, and a plurality of thin film transistors connected to the gate lines, the data lines, and the pixel electrodes for switching signals from the data lines to corresponding pixel electrodes in response to signals transmitted from the gate lines. An alignment layer is formed over the first substrate and is rubbed to align the liquid crystal layer in a predetermined manner.
Generally, the second substrate (color filter array substrate) supports a black matrix layer that prevents light from being transmitted in regions corresponding to areas outside the pixel regions of the first substrate, an R/G/B color filter layer for selectively transmitting predetermined wavelengths of light, and a common electrode for producing images. An alignment layer is also formed over the second substrate and is rubbed to align the liquid crystal layer in a predetermined manner.
Upon applying predetermined voltages to the pixel and common electrodes of the LCD panel described above, an electric field, vertically oriented with respect to the first and second substrates, is generated to alter an arrangement of liquid crystal molecules within the liquid crystal layer. Upon altering the arrangement of liquid crystal molecules, light transmittance characteristics of the LCD panel are selectively altered and an image can thus be expressed. The LCD panel described above has a high aperture ratio but undesirably displays images over a narrow range of viewing angles. To solve this problem, In-Plane Switching (IPS) mode LCD devices have been developed, in which common and pixel electrodes are both formed on the first substrate. Accordingly, IPS mode LCD devices generate electric fields oriented parallel to the substrates.
FIG. 1 illustrates a plan view of one pixel of a first substrate in a related art IPS mode LCD device. FIG. 2 illustrates a cross-sectional view of a first substrate and an opposing second substrate taken along line I-I′ of FIG. 1.
Referring to FIGS. 1 and 2, a plurality of gate and data lines 12 and 24, respectively, are formed on a first substrate 10a so as to cross each other and define a plurality of pixel regions P. A common electrode line 16 is formed adjacent to, and parallel with, the gate lines 12. A thin film transistor T, including a gate electrode 14, an active layer 20, a source electrode 26, and a drain electrode 28, is formed at a crossing of the gate and data lines 12 and 24, wherein the source electrode 26 extends from the data line 24, and the gate electrode 14 projects from the gate line 12. A pixel electrode 30 is formed within the pixel region P so as to be parallel with the data line 24 and electrically connected to the drain electrode 28. The pixel electrode 30 includes a plurality of first pixel electrode parts and one second pixel electrode part each arranged over the common electrode line 16. The first pixel electrode parts are spaced apart from each other at a fixed interval and extend along a first direction over the first substrate while the second pixel electrode part electrically connects the first pixel electrode parts to each other. Further, a common electrode 17 is formed between, and parallel to, each of the first pixel electrode parts so as to be electrically connected to the common electrode line 16. The common electrode 17 includes a plurality of first common electrode parts arranged between the first pixel electrode parts and extending away from the common electrode line 16 in addition to one second common electrode part electrically connecting the first common electrode parts to each other. Outermost ones of the first common electrode parts within the pixel region P are spaced apart from the data line 24 by a predetermined distance. Additionally, insulating layers 21 are formed between the data and common electrode lines 24 and 16, respectively, and between the common electrode line 16 and the common electrode 17. Lastly, a first alignment layer 60a is formed over the entire surface of the first substrate 10a, including over the pixel electrode 30.
As shown in FIG. 2, the second substrate 10b opposes the first substrate 10a and supports includes a black matrix layer 42, an R/G/B color filter layer 44, an overcoat layer 52, and a second alignment layer 60b. The black matrix layer 42 prevents light from being transmitted in regions corresponding to areas outside the pixel regions of the first substrate 10a and the R/G/B color filter layer 44 dimensionally conforms to the pixel region P and selectively transmits predetermined wavelengths of light. The overcoat layer 52 is formed on the color filter layer 44 to planarize the topography of the color filter layer 44 and to prevent pigments within the color filter layer 44 from diffusing into the liquid crystal layer 50, formed between the first and second substrates 10a and 10b. Lastly, the second alignment layer 60b is formed over the entire surface of the second substrate 10b, including over the overcoat layer 52.
The first and second alignment layers 60a and 60b discussed above align liquid crystal molecules within the liquid crystal layer 50 along predetermined directions and are typically formed of a polymer material such as polyamide, polyimide compound, PVA (Polyvinyl Alcohol), polyamic acid, or a photosensitive material such as PVCH (PolyvinylCinnamate), PSCN (PolysiloxaneCinnamate) or CelCN (CelluloseCinnmate)-based compound. The overcoat layer 52 discussed above is typically formed from any one of a photosensitive resin or an acrylic resin.
Upon driving LCD devices having a common electrode formed over the entire surface of the second substrate, including over the black matrix and color filter layers, an electric field is generated that is vertically oriented with respect to the surface of the first and second substrates. Accordingly, the common electrode is functionally equivalent to the overcoat layer 52 illustrated in FIG. 2. However, upon driving IPS mode LCD devices, an electric field is generated that is parallel to the surface of the first and second substrates. Because the common electrode 17 is formed on the first substrate 10, it is necessary to form the overcoat layer 52 on the second substrate 10b. 
A related art method for fabricating the second substrate 10b shown in FIG. 2 will now be described in greater detail with respect to FIGS. 3A to 3C.
Referring to FIG. 3A, a light-shielding layer is deposited on the second substrate 10b and selectively removed in regions corresponding to the pixel regions P (i.e., patterned) to form the black matrix layer 42. Next, photosensitive layers (not shown) are repeatedly deposited over the entire surface of the second substrate 10b, including over the black matrix layer 42, and patterned to remain within predetermined portions of the pixel regions P to form the R/G/B color filter layer 44. After the color filter and black matrix layers 44 and 42 are formed, a cleaning process is performed to remove foreign materials from the second substrate 10b. 
Referring to FIG. 3B, a photosensitive resin or acrylic resin is coated (i.e., spin coated) over the entire surface of the second substrate 10b, including over the color filter and black matrix layers 44 and 42. The coated resin is then cured to form the overcoat layer 52.
Referring to FIG. 3C, after forming the overcoat layer 52, a polymer material such as polyamide, polyimide compound, PVA (Polyvinyl Alcohol), polyamic acid, or a photosensitive material such as PVCH (PolyvinylCinnamate), PSCN (PolysiloxaneCinnamate) or CelCN (CelluloseCinnmate)-based compound is coated (i.e., spin coated) over the entire surface of the second substrate 10b, including over the overcoat layer 52, cured, and rubbed or irradiated with light to form the second alignment layer 60b. 
Fabricating the related art IPS mode LCD device as described above, however, is disadvantageous because the overcoat layer and the second alignment layer must be formed using separate coating (i.e., spin-coating) processes. The necessity for separate process steps undesirably increases the fabrication time and cost. Further, the minimum thickness of each of the overcoat and second alignment layers is determined by the characteristics of the coating processes (i.e., spin-coating processes) by which they are formed. Therefore, the combined thickness of the overcoat and second alignment layers can become undesirably thick and disadvantageously reduce the light transmittance characteristics of the IPS mode LCD device.